Hybrid type inflators used for inflating airbag systems are filled with an inert gas in the inflator chamber that is heated by a pyrotechnic within a heater structure. This pyrotechnic is activated generally by firing a squib or igniter means for initiating a rapid burning of the generant within a heater system; this generant heats the inert gas such that sufficient pressure can be achieved prior to being released through the airbag system thereby inflating it.
U.S. Pat. No. 5,660,412 discloses the use of a primary heater and a bypass heater to achieve more rapid heating of the pressurized fluid in a cylindrical hybrid gas generator. The patent teaches a first pyrotechnic material as a primary source of heating and a second pyrotechnic material having a faster rate of gas generation wherein portions of these products of combustion directly heat the inflation gas using a single initiator.
More typically multi stage hybrid inflators utilize two squibs or initiators to separately ignite two separate generants or pyrotechnic materials. Hybrid inflators of this type are disclosed in US 2004/0100079 A1, U.S. Pat. No. 6,793,244 B1, US 2003/0001370 A1 and U.S. Pat. No. 6,488,310 B1. In each of these prior art references the gas generant is contained in a heater element that can be activated in multiple stages by using two separate squibs or initiators. These firing sequences are made possible by time delay such that when one activates the initiators they can be activated singularly or in parallel or sequentially.
The use of two separate heaters in opposite ends of the cylindrical inflator with the central exhaust has been taught in U.S. Pat. No. 6,253,683 B1 and EP 1 405 775. These early prior art cylindrical inflators with a central exhaust did not employ bypass enhancer heaters to pre-heat the inert gas.